Is this something we should keep on the shelf?From my perspective, all you have to do is be out riding around and lose (disintegrate) your Ujoint to decide it is a good part to have a spare of (I have that Tshirt). Especially with two valks. And the fact they were unavailable a few years back, for the better part of a year getting a new supplier and back in the pipeline is another reason to have a spare. (And remember, when they did become available again, they were now available at $130 (or so), rather than the previous $65 (or so).
And a coverset (or Pingle), a kickstand bolt and spring, brake pads, battery, Orings, front tires, yada.
However, never having lost an alternator, I have no spare.

Funny how that works.
And BTW, just logging into a parts supplier, and adding a part to cart, but not checking out (or even checking out) is no guarantee that the part is actually in stock. Their websites are supposed to be kept up to date on backordered/out-of-stock stuff, but you can't really depend on that. This is really true of
all web sellers.